Not Applicable.
Not Applicable.
This invention relates to laboratory work cabinets. More specifically, this invention relates to laboratory work cabinets which provide improved exhaust containment such that the cabinets may be operated at lower inflow air velocities than traditional work cabinets, thereby reducing the energy required to condition the exhausted air.
Laboratory work cabinets, or xe2x80x9cfume hoodsxe2x80x9d, are ventilated enclosures where undesirable and dangerous fumes or vapors are captured, contained and removed. These fumes or vapors are prevented from escaping into the external laboratory environment and, instead, are diluted with room air and exhausted through the hood""s exhaust system where they can be adequately dispersed. This capture and containment is accomplished by controlling gas and vapor contaminants present in the hood work area and directing them away from the user.
Fume hoods known in the prior art generally comprise a cabinet which defines an enclosed work area. The cabinet has an opening in the front face thereof for providing access to the work area and typically includes a panel or sash which is movable between open and closed positions to provide selective access to the work area. The sash is movable between various heights to accommodate positioning of hands and arms in the work area. Prior art fume hoods typically include an exhaust system, often connected to the top of the cabinet, for venting fumes that collect in the work area of the cabinet.
Air flow into prior art fume hoods, or xe2x80x9cexhaust hoodsxe2x80x9d, typically is achieved by an exhaust blower which xe2x80x9cpullsxe2x80x9d air from the external laboratory environment into and through the hood and its associated exhaust system. Thus, contaminants are drawn away from the operator. This pull of air into the hood is measured as face velocity. It is, of course, essential to provide an adequate face velocity for any laboratory fume hood so as to ensure containment of the fumes or other contaminants and to ensure that these contaminants will ultimately be removed through the exhaust system. The face velocity, however, must not be so high that it creates turbulent conditions within the hood which can lead to the escape of contaminants. Accordingly, it is desirable that the face velocity be maintained nearly constant, not varying appreciably throughout the normal working range of the sash. Since raising of the sash increases the effective size of the exhaust hood opening, the volume of air pulled through the face opening of the hood must be increased in order to maintain a constant face velocity as the sash is raised. For most materials that are handled in fume hoods, a face velocity of approximately 100 feet per minute (fpm) is satisfactory.
Fume hoods known in the prior art often incorporate a bypass opening located in the front face of the cabinet above the opening into the work area, or located in the top face of the cabinet in the area at or near the front face. Bypass hoods are designed so that as the sash is moved toward a closed position, air which normally enters the hood through the sash opening is redirected through the bypass opening thus reducing fluctuations in face velocity as the position of the sash is varied. Therefore, the possibility that the velocity will reach a level that would be detrimental to the procedures being performed in the work area, or to those persons in the vicinity of the work area, is reduced.
Bypass hoods are not without weaknesses, however. For instance, the angle at which bypass air enters the work area changes with sash position and the face velocity may increase to as much as three times the normal face velocity as the sash moves toward the closed position even though air is being directed through the bypass opening.
Additionally, prior art fume hoods, including bypass hoods, exhibit characteristic internal vortex air flows known as xe2x80x9ca rollxe2x80x9d in which a portion of the incoming air flow rolls up the interior side of the rear face and down the interior side of the sash. Fumes generated within the hood from laboratory procedures often are entrained into the roll resulting in an increase in the concentration of contaminants throughout the work area. Specifically, this tendency for contaminated air to roll forward produces high concentrations of contaminants in the area directly behind the sash increasing the opportunity for leakage at the sash handle.
A primary factor in creating this undesirable air flow is that traditional fume hoods are unable to remove contaminants from the work area as quickly as they are generated, i.e., contaminants are not removed on the xe2x80x9cfirst passxe2x80x9d. Rather, fumes generated by laboratory procedures freely mix with incoming air, circulate in the vortex and come back down the interior surface of the sash. This raises the parts per million concentration of the contaminant throughout the work area.
A further weakness of prior art fume hoods is that air flow within the hood interior varies with the position of the sash making it difficult to idealize conditions for optimum containment. Still further, bypass air, while beneficial to maintaining a constant face velocity, continuously varies with sash position, both in volume and direction, as influenced by the changing internal conditions. This makes the hood less robust in its ability to contain contaminants. Additionally, while traditional and bypass fume hoods effectively contain contamination behind the vertical plane of the sash, they are susceptible to external conditions, traffic patterns near the sash opening and work procedures, as contaminants are concentrated directly behind the plane of the sash and under the sash handle. Another weakness of prior art fume hoods is their susceptibility to lead around the front sash foil. Existing air foils are not designed to provide uniform velocities both with and without a person standing in front of the hood.
Because of the afore-described deficiencies, typical prior art fume hoods are designed to provide a certain excess air flow so that even during more optimal operating conditions there will be adequate air flow to satisfy safe operating conditions. This design criteria results in significant energy loss both through exhaust system power requirements and by removing conditioned air from the building which must be replaced by other conditioned air.
Accordingly, there remains a need for a laboratory work cabinet which more effectively contains contaminated air and is less susceptible to external conditions such as traffic patterns near the sash opening or air fluctuations caused by work procedures. Further, there remains a need for a laboratory work cabinet which substantially removes contaminants from the work area as quickly as they are generated. Still further, there remains a need in the fume hood industry for a laboratory work cabinet which operates at lower inflow air velocities than traditional hoods thereby reducing the energy required to operate the exhaust system and reducing the demand on the building HVAC system.
Accordingly, in one of its aspects, the present invention provides a laboratory fume hood which more effectively contains contaminated air and is less susceptible to external conditions.
In another of its aspects, the present invention provides an exhaust hood in which fumes are contained deeper into the interior of the hood reducing contaminant concentrations directly behind the plane of the sash.
In yet another aspects, the present invention provides an exhaust hood which operates at lower inflow air velocities than traditional hoods thereby reducing the energy required to exhaust air at acceptably low contaminant concentrations and reducing the demand on the HVAC system.
In another of its aspects, the present invention provides a laboratory fume hood which substantially removes contaminants from the work area as quickly as they are generated.
In an additional aspect, the present invention provides a laboratory fume hood which substantially contains contaminants in the area of the source so that the contaminants are not mixed throughout the total interior air volume.
In a further aspect, the present invention provides a laboratory fume hood which permits largely laminar air flow moving through the sash opening and directly into the exhaust system. This flow minimizes the xe2x80x9crollxe2x80x9d or vortex typical in fume hoods of the prior art.
According to the present invention, the foregoing and other aspects are achieved, in one embodiment, by an exhaust hood which comprises a cabinet, an exhaust plenum and a vertically mounted sash. The exhaust plenum is located at a rear face of the cabinet and is defined by the rear face and a multi-structural baffle. Each structure of the baffle defines a plenum zone through which air entering an opening in a front face of the cabinet enters the plenum. In other words, air entering through a lower portion of the opening primarily enters the plenum through a first plenum zone and air entering through an upper portion of the opening primarily enters the plenum through a second plenum zone, the second zone being located above the first zone. Optionally, the baffle may contain a third structure defining a third plenum zone that is located between the first and second zones. In this embodiment, air entering an intermediate portion of the opening primarily enters the plenum through the third zone.
The vertically mounted sash may be adjusted to various heights thereby changing the size of the opening in the front face. When the sash is raised to a first height, air flows primarily across the work area and into the first structure. When the sash is raised to a second height, air entering the lower portion of the opening flows primarily across the work area and into the first structure and air entering the upper portion of the opening flows primarily across the work area and into the second structure. Optionally, the sash may be raised to a third height whereby air entering the intermediate portion of the opening flows primarily across the work area and into the third structure. The flow of air selectively entering vertically spaced plenum zones results in a substantially laminar flow of air through the work area.
Aspects of the present invention are further achieved by a sash handle for use with an exhaust hood sash which is spaced from the sash and thus permits the passage of air between the sash and the handle. The handle of the present invention comprises a slotted or perforated rear plate, an upper arm portion which extends outwardly and upwardly from an upper edge of the plate and a lower arm portion which extends outwardly and upwardly from a lower edge of the plate. The shape and positioning of the upper and lower arm portions forms a cavity between the arm portions and the rear plate which guides the passage of air at the same velocity as the inflow air to the primary hood opening thereby sweeping contaminated air away from the opening without creating turbulence.
Aspects of the present invention are further achieved by an air foil for use with an exhaust hood, the air foil being positioned in the area of the front edge of the bottom surface of the hood. The foil of the present invention includes a curved plate having a plurality of apertures therein, the plate extending from the front edge of the bottom surface toward the work area defined by the hood. The plate terminates at a position spaced from the bottom surface. As such, air is allowed to sweep the floor of the work area and across the surface of the airfoil at higher speed than would otherwise be possible by entering through the apertures in the plate and exiting into the work area through the space between the plate and the bottom surface. The air flow across the work surface is enhanced by a radiused front edge on the surface to optimize air speed.
Further aspects of the present invention are achieved by an air stabilizing device for use with an exhaust hood cabinet which includes first and second dividers which are spaced from one another and extend outwardly from a rear face of the hood and into the work area defined thereby. This placement of dividers permits air to be contained between the dividers and aids in preventing its permeation into other areas of the hood.
A feature of the invention is the incorporation of a deflector for controlling the direction of bypass air which passes across the inside of the sash when the latter is closed or partially closed and to direct air into the top of the work area to eliminate the build of contaminants.
Additional aspects of the present invention are achieved by an air flow regulating device for use with an exhaust hood which includes one or more side shields extending at an angle between the vertical side faces of the hood and the front face thereof. Each side shield has apertures therein permitting air to flow through the shields. However, positioned at an angle as taught by the present invention, a portion of the air that would otherwise enter the opening in the front face is prevented from entering the work area and thus less air fluctuation occurs in the work area.
Aspects of the present invention are further achieved by a floor for use with an exhaust hood which includes a plurality of elongated U-shaped channels which are aligned substantially parallel to the vertical side faces of the hood. Accordingly, air is allowed to flow beneath the work surface as it may flow through the channels toward the rear plenum. Optionally, the floor may be removable. Thus, if a user spills a liquid onto the floor of the hood, much of the liquid will accumulate in the channels and away from the working surface. The spill is then easier to clean as the floor may be removed and washed away from the hood itself.
Additional aspects of the invention, together with the advantages and novel features appurtenant thereto, will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned from the practice of the invention. The aspects and advantages of the invention may be realized and attained by means, instrumentalities and combinations particularly pointed out in the appended claims.